Pneumatic Cylinder Air Consumption (SCFM)

Compressed air is billed in STANDARD cubic feet -- free air at atmospheric pressure -- and a cylinder is filled with COMPRESSED air, so every cubic foot of cylinder volume at 90 psig took just over seven cubic feet of free air to fill, and every cycle throws all of it away through the exhaust port. That factor of seven is the whole reason pneumatics are expensive to run and the reason a shop's compressor is always smaller than its air demand. The rod side matters and is easy to skip: on the retract stroke the rod occupies part of the bore, so the retract volume is smaller than the extend volume, and on a large-rod cylinder that difference is substantial -- counting both strokes at full bore over-estimates, counting only the extend stroke under-estimates by nearly half. A 2.5 in bore with a 1 in rod on a 12 in stroke cycling 20 times a minute at 90 psig moves 108 cubic inches per cycle and consumes 8.9 SCFM, roughly two horsepower of compressor running continuously for one small actuator; ten of them is a 20 hp compressor doing nothing but cycling cylinders. The pressure lever is real and underused: dropping the supply from 90 psig to 70 psig cuts the compression ratio from 7.12 to 5.76 and the consumption by 19%, which is free money if the application still makes its force at the lower pressure. Demand only. This does not size a compressor, a receiver, or the distribution piping, and it takes no account of the leakage that in most shops exceeds the productive demand, of valve and fitting losses, of the air a cylinder's cushions and pilot lines consume, or of the dryer and filtration load the flow implies. Duty cycle is assumed steady at the entered rate. The compressor manufacturer's rating at the actual discharge pressure, and an air audit of the real system, govern.

extend = pi (bore/2)^2 x stroke; retract = (pi (bore/2)^2 - pi (rod/2)^2) x stroke; volume per cycle = the sum / 1728; compression ratio = (psig + 14.7) / 14.7; SCFM = volume per cycle x cycles per minute x compression ratio x cylinders; compressor hp = SCFM / CFM-per-hp.

Cylinder swept volume from bore, rod, and stroke, and the compression ratio (gauge pressure + 14.7) / 14.7 that converts compressed volume to FREE air, by name. Demand only: no compressor, receiver, piping, or leakage. The compressor manufacturer's rating at the actual discharge pressure, and an air audit of the real system, govern.

Geometry and the ideal-gas pressure ratio on the user's own cylinder dimensions and supply pressure; no manufacturer consumption table is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: bore_in, rod_in, stroke_in, cycles_per_min, pressure_psig, cylinders, cfm_per_hp, extend_in3, retract_in3, per_cycle_in3, volume_per_cycle_ft3, compression_ratio, scfm_total, compressor_hp

• Standard versus compressed SCFM is free air; the compression ratio converts, and forgetting it understates demand about sevenfold at 90 psigcompressed air practice
• The rod side is smaller the rod displaces part of the bore on retract; counting both strokes at full bore over-estimatescylinder geometry
• Leakage is not counted in most shops leakage exceeds productive demand; an air audit governscompressed air practice